Is his love for a girl that made him stop committing forbidden acts unlawful or lawful, and what should he do if it is unlawful?

Love before marriage is impermissible. However, a mere attachment of the heart to a girl, without any deliberate action on your part, and as long as it does not exceed the bounds of moderation, incurs no blame. But it is not permissible to establish a relationship. Rather, the prescribed action, if the girl is religious, is to propose to her, based on the Prophet's (peace be upon him) saying: "We have not seen for two who love each other anything better than marriage." If you are unable to marry, or if it is not possible for you to marry her, then you must strive to remove this attachment, occupy yourself with what benefits you, fast frequently, and seek Allah's help until marriage becomes possible for you.